The invention relates to a method and apparatus for correcting registration faults based on changes in machine state in a multicolor printing machine having a number of printing units with image cylinders and exposure devices for the digital production of color separations on the image cylinders, the actions of setting up and combining the color separations being controlled such that in-register prints are achieved.
The invention also relate to an apparatus for implementing the method with a multicolor printing machine having a number of printing units with image cylinders and equipment for the digital production of color separations on the image cylinders, and at least one controller to achieve in-register prints, the actions of producing and combining the color separations being controlled.
Printing colored illustrations, in particular color images, is carried out by a number of color separations being printed over one another. These are generally the colors yellow, magenta and cyan as well as black. If required, special colors are added. By overprinting these colors, all color combinations can be achieved, the quality of the prints depending significantly on the in-register overprinting of the color separations. In the case of digital printing processes, for example, electrostatic printing processes, the maintenance of the registration of the overprint is achieved by the image production equipment being controlled such that the color separations meet one another in-register when they are transferred to a printing substrate.
A method and apparatus of a printing machine are disclosed by U.S. Pat. No. 5,715,498. There, the correction of registration faults by the print and the registration of registration marks is proposed. Registration faults are often caused by changes in machine state. For example, temperature changes lead to dimensional changes, in particular with respect to the distance between the individual printing units or between the bearings of various cylinders. The disadvantage of this technical solution is that, in the event of changes in machine state, counteractive control is always only possible for the next print, since the effect of a change in machine state is only registered when it has already caused a registration fault. As a result, test prints are necessary and time is lost, which in the case of small jobs is often uneconomic and, therefore, not acceptable.
The invention is, therefore, based on the object, in a method and apparatus for printing, of preventing the occurrence of registration faults as a result of dimensional changes, in particular between the printing units or between mountings of various cylinders, to the largest possible extent from the start.
According to the invention the object is achieved by changes in machine state being registered and, on the basis of these changes in machine state, the production of the color separations being changed in such that a disturbance to the registration accuracy of the print, threatened by a change in machine state, is compensated for.
The advantage of the invention is that many changes, which influence registration, are counteracted without any delay. In this way, an influence of a change in machine state can be counteracted very quickly such that test prints and/or registration marks are generally not necessary. It is precisely in the case of small jobs that this is of economic importance, since machine time and often also printing substrates can be saved.
However, it is, of course, possible for the measure according to the invention to be further combined with further registration detection systems, control systems and regulating systems, in order to achieve monitoring and, if necessary, more exact correction, in particular in the case of further fault causes. In this case, the advantage according to the invention, that significant fault causes can be compensated for without any delay, is still maintained, and faults are avoided or at least considerably reduced. Reject prints can be avoided to the greatest possible extent in this way, and no other time-consuming control or regulating operations are needed either in order to achieve usable prints.
By the invention, various types of changes in machine state can be registered and taken into account. The aim is to register those changes in machine state, which have an influence on registration. These are primarily dimensional changes, in particular between the individual printing units, but also between the mountings of various cylinders, such as between image cylinders and image transfer cylinders. Such changes in spacing are caused by temperature changes and/or changes in mechanical stresses.
It is possible that the temperature of the multicolor printing machine is registered at at least one location. Further possibilities are to register the mechanical stress or a change in spacing in the multicolor printing machine. Measurements are expediently made at a number of locations, the measurement locations being placed such that the changes in machine state are registered at the locations at which they exert an influence on the registration, which is critical for the print quality, such as the spacing between the individual printing units. With respect to the apparatus, it is then proposed that at least one temperature sensor, at least one sensor for measuring mechanical stresses and/or at least one sensor for registering dimensional changes be provided.
For the compensation of the threatened disturbance to registration accuracy, there are various possibilities. One proposal is that a change in machine state be registered and a disturbance to the registration accuracy of the print, threatened thereby, be countered by a controlled change in the production of the color separations, and then the registration be regulated on the basis of registration of the printed result. In this case, counteractive control precedes the regulation, in order to minimize the magnitude of the threatened registration fault from the start. Another possibility is for a change in machine state to be registered quantitatively and a disturbance to registration accuracy of the print, threatened thereby, to be compensated for, on the basis of the change in machine state, by stored values based on experience, which are assigned to the quantitative change in machine state.
With respect to the apparatus of this invention, it is proposed that a controller be provided that in the event of a change in machine state counters a disturbance to registration accuracy of the print, threatened thereby, by a controlled change in the production of the color separations, and then regulates the same on the basis of registering the printed result. When registering the printed result, a print itself, for example, the start of the print, can be registered, or it is possible that a sensor for detecting registration marks is arranged in order to register the printed result. Registration marks can be printed on the carrier for printing substrates or on an image-free edge of a printing substrate, for example, on a test sheet. For the other alternative with respect to compensating for the threatened disturbance to registration accuracy, it is proposed that at least one sensor be provided for the quantitative registration of changes in machine state, and at least one controller be provided that it effects compensation for a disturbance, threatened thereby, to registration accuracy of the print by performing the control on the basis of stored values based on experience, which are assigned to the quantitative changes in machine state.
Of course, the measure according to the invention does not rule out the combination with a conventional registration regulating system; it is generally an extension to such, in order to counteract effects of a change before they manifest themselves in registration faults. Provision can, therefore, be made for a controller to be provided in addition to the registration of registration marks by a sensor or another registration regulating system or control system. However, if other fault causes are few, then the registration regulating system according to the invention can also on its own lead to the target of high registration accuracy.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.